The R-T-B based sintered magnet (R represents a rare earth element, T represents at least one iron family element with Fe as essential, and B represents boron) represented by the Nd—Fe—B based sintered magnet has a high saturation magnetic flux density. Thus, it is useful for the minimization and efficiency improvement of the equipments used and can be used in a voice coil motor of a hard disk drive. Recently, such magnets are also applied to motors in various fields or drive motors for hybrid vehicles. From the view point of energy saving or the like, it is desired that more such magnets will be used in these fields. However, during the application of R-T-B based sintered magnets in hybrid vehicles or the like, the magnets are exposed to a relatively high temperature. In this respect, it is important to inhibit the demagnetization at a high temperature caused by heat. Further, it is well known that the demagnetization at a high temperature can be effectively inhibited by sufficiently improving the coercivity (Hcj) of the R-T-B based sintered magnet at room temperature.
For example, as a well known method for improving the coercivity of the Nd—Fe—B based sintered magnet at room temperature, part of Nd in the Nd2Fe14B compound (which is the main phase) is replaced with the heavy rare earth element(s) such as Dy or Tb. The magneto-crystalline anisotropy constant can be improved by replacing part of Nd with the heavy rare earth element(s). As a result, the coercivity of the Nd—Fe—B based sintered magnet at room temperature can be improved sufficiently. Besides the replacement of heavy rare earth element(s), the addition of Cu or the like will also elevate the coercivity at room temperature (Patent Document 1). The addition of Cu will render Cu form, for example, the Nd—Cu liquid phase in the crystal boundary so that the crystal boundary will become smooth. In this way, the reverse magnetic domains can be prevented from generating.
On the other hand, Patent Documents 2, 3 and 4 have disclosed a technology that the crystal boundary phase (which is the microstructure of the rare earth based magnet) is controlled to improve the coercivity. It can be known from the drawings of these Patent Documents that the crystal boundary phases refer to the crystal boundary phases surrounded by three or more main phase crystal grains and are also called the triple junction points. In Patent Document 2, a technology has been disclosed for forming two kinds of triple junction points with different Dy concentrations. That is, it has been disclosed that crystal boundary phases (triple junction points) are formed with part areas having a high concentration of Dy and the total concentration of Dy unchanged so that a high resistance with respect to the reversal of the magnetic domain can be maintained. The Patent Document 3 has disclosed a technology that three kinds of crystal boundary phases (triple junction points) (the first one, second one and third one) are formed with different total atomic concentrations of rare earth elements, wherein the atomic concentration of rare earth elements in the third crystal boundary phase is lower than that in other two crystal boundary phases, and the atomic concentration of Fe in the third crystal boundary phase is higher than that in other two crystal boundary phases. In this way, a third crystal boundary phase with a high Fe concentration can be formed in the crystal boundary phases, resulting in the improvement of coercivity. In addition, Patent Document 4 has disclosed an R-T-B based rare earth based sintered magnet which is formed by a sintered body, and the sintered body consists of main phases (which mainly contains R2T14B) and crystal boundary phases with more R than the main phases. The crystal boundary phases contain phases with the total atomic concentration of rare earth elements being 70 atomic % or more and phases with the total atomic concentration of rare earth elements being 25 to 35 atomic %. The phases with the total atomic concentration of rare earth elements being 25 to 35 atomic % are referred to as the transition metal-rich phases, and the atomic concentration of Fe in these phases are preferably 50 to 70 atomic %. In this respect, coercivity is improved.